1. Field of the Invention
The present invention relates generally to organic thin film transistor (OTFT) technology and, more particularly, to a low-voltage OTFT having a gate dielectric layer of ultra-thin metal oxide self-grown by direct oxidation of a metal gate electrode in O2 plasma process or having a dual gate dielectric layer composed of a self-grown metal oxide layer and an organic dielectric layer.
2. Description of the Related Art
In these days organic semiconductor such as pentacene has been widely studied. Organic semiconductor is produced by various synthesis ways and easily formed in the shape of fiber or film. Additionally, organic semiconductor has good flexibility, good conductivity, and relatively low cost of production. Thanks to these advantages, organic semiconductor is studied as a new material available for wide areas including electronic devices and optical devices.
The OTFT employs organic semiconductor for semiconductor regions in comparison with typical silicon TFT using amorphous silicon. Though being similar in structure to the typical silicon TFT, the OTFT has merits in fabrication such as simpler processes and lower cost. For such reasons, new attempts to apply OTFT technology to advanced electronic applications, including flexible display, radio frequency identification (RFID), and a great variety of portable devices, continue today.
However, modern OTFT technology may have some technical problems to be solved. One of them is to develop a new process of forming a gate dielectric layer at a lower temperature. Silicon oxide or silicon nitride conventionally used as the gate dielectric layer may be formed at higher temperature, thus being not applicable to a glass or plastic substrate.
Another problem with the existing OTFT is to reduce an operating voltage. Low power consumption is prerequisite to applications such as flexible display and RFID, however the OTFT often exceeds 20V. This is due to a relatively thick gate dielectric layer, which commonly reaches 100 nm or more.
Various approaches to solve these problems have been introduced in the art. For example, U.S. Pat. No. 6,207,472 discloses that a gate dielectric layer is formed of Ta2O3, V2O3, TiO2, etc. at 25˜150° C. by using sputtering, spinning, etc. In another case, Korean Published Application No. 2005-31858 discloses an Al2O3 gate dielectric layer deposited by sputtering at a room temperature to about 100° C. In yet another case, Japanese Published Application Nos. 2003-258260 and 2003-258261 disclose anodizing a gate electrode of Ta, Al, etc. to form a gate dielectric layer.
Unfortunately, although these conventional techniques provide their own ways of forming metal oxide as a gate dielectric layer of OTFT at a relatively low temperature, they fail to suggest a way of reducing the thickness of the gate dielectric layer. Gate dielectric thickness is approximately 0.5 μm in case of U.S. Pat. No. 6,207,472 and is between 61 nm and 450 nm in case of Korean Published Application No. 2005-31858. In case of Japanese Published Application Nos. 2003-258260 and 2003-258261, the thickness is described as 85.64 nm, for example.
On the other hand, researches to realize a thinner gate dielectric layer have been continuously carried out in the art. For example, a paper, “Low-voltage organic transistors with an amorphous molecular gate dielectric, Marcus Halik et al., Nature, vol. 431, 2004, pp. 963-966” teaches a 2.5 nm-thick molecular self-assembled monolayer (SAM) gate dielectric on a heavily doped silicon substrate. However, this may lack commercialization since there is no plan to electrically isolate discrete devices under the circumstances the heavily doped silicon substrate is used for gate electrodes.
Another paper, “One volt organic transistor, L. A. Majewski et al., Adv. Mater. 2005, 17, No. 2, pp. 192-196” proposes anodization of metal to form a metal oxide with a thickness of several nanometers as a gate dielectric layer. This may also lack commercialization since anodization, a kind of wet process, may invite an unfavorable peeling of metal.